Interrelationships among body mass, thyrotropin, thyroid hormones, and thyroid-hormone binding proteins in healthy 70-year-old men.

In a previous study of a representative population of 460 men of age 70, we found that the free thyroxin (free T4) concentration was negatively correlated to body mass. In the present study we analyzed other indicators of thyroid function in relation to body mass and body mass index in 181 individuals without any disease or medication influencing thyroidal homeostasis or body mass. There was a significant negative relationship between body mass and body mass index and free T4, total T4, and 3,3',5'-triiodothyronine (rT3). Thus, individuals with high body mass normally had lower circulating concentrations of these hormones than lean individuals, whereas there was no such relationship for thyrotropin or 3,5,3'-triiodothyronine (T3). The thyroid-hormone binding proteins (thyroxin-binding globulin, prealbumin, and albumin) did not correlate with body mass or body mass index. As expected, thyroxin-binding globulin correlated with T4, T3, and rT3, but not with free T4 or thyrotropin. Concentrations of serum albumin correlated with T3 and rT3 concentrations but not with free T4, T4, or thyrotropin concentrations. Thyrotropin concentrations correlated negatively with free T4, T4, and rT3, but not with T3, indicating that circulating T4 plays the dominant role in feedback regulation of thyrotropin in healthy individuals.